The invention relates to the general field of integrated circuits with particular reference to chemical mechanical polishing and avoidance of dishing problems.
Chemical mechanical polishing (CMP) is widely used as a method for planarizing surfaces since the advancing etch front associated with CMP tends to be independent of both the topography and compositional variations within the surface being treated. However, since the pad used for the CMP is soft, it has a slight tendency to follow concavities with the result that, over time, dishing occurs over soft or open areas.
Referring now to FIG. 1, we see there a substrate 11 whose upper surface includes a number of trenches or via holes such as 13. A temporary depression 12 has been formed and partially filled with metal layer 14. Sufficient metal was used to overfill the vias so that the surface 15 of the metal layer was about 2 microns above the floor of 12.
FIG. 2 shows the same structure after conventional CMP according to the practices of the prior art. As intended, the thickness of the metal layer 14 has been reduced above all surfaces outside the trench. However, in addition, the metal surface 25 inside the trench has not been left entirely untouched, as was intended. Instead, some dishing has occurred together with some chemical erosion of the metal.
The present invention discloses a solution to this problem that prevents dishing and erosion without affecting the main process.
A routine search of the prior art was performed. The following references of interest were found: In U.S. Pat. No. 5,618,381, Doan et al. show a CMP process that uses a protective layer that polishes significantly more slowly than the metal being planarized. This causes material in the crests to be removed faster than material in the valleys, thereby enhancing planarization. Cheung, in U.S. Pat. No. 6,056,864, shows an ECD copper planarization process where the copper is wet etched or electrolytically etched and the subjected to CMP. In U.S. Pat. No. 5,885,900, Schwartz shows a global planarization process for conductive layers using a wet etch back step while Joshi et al. (U.S. Pat. No. 5,731,254) show using a CuGe hard cap during CMP.
It has been an object of the present invention to provide a process for chemical mechanical polishing of a surface that includes a dense trench array filled with metal.
Another object has been that said process lead to little or no dishing of the trenches.
A further object has been that said process lead to little or no erosion of the metal in the trenches or of the surrounding oxide.
These processes have been achieved by first covering the trenches with a material whose polishing rate under CMP is similar to that of the metal in the trenches. Spin-coating is used so that only the depression in which the trenches lie gets filled. After CMP, any residue of this material is removed leaving behind a surface that has been planarized to the intended extent without the introduction of significant dishing and with minimum erosion of the metal.